The present invention relates to a semiconductor laser diode of a multibeam structure for emitting laser beams in parallel, its manufacture method and a manufacture method for an opto-semiconductor device. The present invention relates to technologies effective for being applied to manufacture technologies of e.g., a ridge structure semiconductor laser diode.
A semiconductor laser (laser diode: LD) is widely used as a light source of an optical communication system and an information processing apparatus. A visual light optical semiconductor laser is used for CD, DVD, a laser printer, POS, a bar code reader as well as a light source of an information processing apparatus such as a document file system.
A semiconductor laser diode (opto-semiconductor device) has a structure that a number of semiconductor layers (multilayer semiconductor layer) are epitaxially grown on a first surface of a semiconductor substrate. An active layer is disposed as a middle layer of the multilayer semiconductor layer. Of two layers sandwiching the active layer, one layer is a semiconductor layer of a first conductivity type and the other layer is a semiconductor layer of a second conductivity type to thereby form a pn junction. In order to form a resonator (optical waveguide) for laser oscillation, various structures have been adopted such as a narrow electrode and a ridge structure. Since the semiconductor laser diode has an anode electrode (p-electrode) and a cathode electrode (n-electrode), there is adopted a structure that the electrodes are disposed on the same plane side of the semiconductor laser diode or a structure that the electrodes are separately disposed on the front and rear surfaces of the semiconductor laser diode.
A semiconductor laser diode of a ridge structure has two separation grooves formed on the surface of a flat semiconductor layer in order to form a ridge (ridge stripe). Therefore, even in a state that an electrode is formed on the surface of the semiconductor laser diode, the separation grooves and steps of the concave and convex portions of the ridge appear on the surface correspondingly. As a result, if the electrode of the semiconductor laser diode is fixed in a so-called junction down state that the electrode plane is stacked upon a submount, with a soft bonding material such as solder being interposed therebetween, a void is likely to be generated between the submount and semiconductor laser diode due to the concave and convex portions. Although heat generated in the semiconductor laser diode is transferred to the submount via the bonding material, if the void is generated, it is difficult to efficiently transfer heat to the submount (e.g., refer to JP-A-2005-217255).
In the invention described in JP-A-2005-217255, the surface of a plated metal layer formed on the surface of a semiconductor laser diode is planarized to prevent a void from being generated.
A semiconductor laser diode of a so-called multibeam structure is known in which a plurality of resonators are disposed in parallel on a single semiconductor laser diode (also called a semiconductor laser chip) (e.g., refer to JP-A-2002-57401). JP-A-2002-57401 discloses a structure (2-beam structure) that two semiconductor laser diodes are mounted on one chip. The semiconductor laser disclosed in JP-A-2002-57401 has a buried hetero structure different from the ridge structure.
The semiconductor laser diode of the buried hetero structure is formed by using a substrate formed with a stripe-shaped mesa projection. The semiconductor laser has a semiconductor lamination body including a first clad layer of a first conductivity type, an active layer, a second clad layer of a second conductivity type, and a current block layer of the first conductivity type formed on mesa grooves on both sides of the mesa projection and contacting both side walls of the active layer over the whole thickness of the active layer. As an upper layer of the semiconductor lamination body, a first electrode of a structure including an alloying preventive film is formed, and a second electrode is formed on the bottom surface of the substrate. A step caused by a height of the mesa projection is formed on the surface of the semiconductor lamination body. The first electrode has a structure having two alloying preventive films and a stress relaxing layer formed between the two alloying preventive films and made of material softer than the two alloying preventive films.
The following problem occurs when mounting a semiconductor laser structure whose first electrode does not have a structure having two alloying preventive films and a stress relaxing layer formed between the two alloying preventive films and made of material softer than the two alloying preventive films.
Namely, for example, when the semiconductor laser is mounted from the first electrode side on a substrate such as a submount by using solder, a stress is applied from a bonding plane of the first electrode and solder to an operation region having a triangular cross sectional shape and constituted of the first clad layer, active layer and second clad layer respectively on the mesa projection. More in detail, a step is left on the surface of a semiconductor lamination body of the first electrode, corresponding to a height of the mesa projection formed on the semiconductor substrate, and a stress caused by a difference of a thermal expansion coefficient between the submount substrate and semiconductor laser is applied to the operation region so that the laser characteristics such as a polarization direction of a laser beam are influenced.
However, the semiconductor laser whose first electrode has two alloying preventive films can prevent alloying at the interface between the electrode and semiconductor lamination body or at the interface between the electrode and solder, in the area of the alloying preventive films. Further, by disposing the stress relaxing layer of Au or the like softer than the alloying preventive films at the position where alloying progress is to be prevented, it becomes possible to relax a stress to be caused by a difference of a thermal expansion coefficient between the substrate and semiconductor laser. As a result, even the structure that two semiconductor laser diodes are mounted on one chip, it is possible to prevent influence upon the laser characteristics such as a polarization direction of a laser beam.
A semiconductor laser diode of a multibeam structure is used as a light source of a plane paper copier (PPC) and a laser beam printer (LBP). A multibeam laser is required that each beam has uniform polarization characteristics. However, in practice, each beam has a variation in characteristics, and mitigating this variation is a technical issue inherent to a multibeam laser.
The variation mainly results from the following. (1) A size variation in a wafer process of manufacturing a semiconductor laser diode generates a variation in characteristics.
(2) A semiconductor laser device is made of compound semiconductor, and on the surface of the device, an insulating film such as an SiO2 film and an electrode made of metal are formed. A stress (inner stress) is generated in the diode (chip) due to a difference of a thermal expansion coefficient between the insulating film and electrode, and this inner stress generates a variation in characteristics.
(3) A semiconductor laser diode is fixed to a submount (supporting base plate) made of aluminum nitride (AlN) and also fixed to a heat sink made of metal such as copper. A thermal stress is generated due to differences of linear thermal expansion coefficients of compound semiconductor, aluminum nitride (AlN) and the like, and metal and the like. This thermal stress generates a variation in characteristics.
The factors (2) and (3) in particular of generating a stress influence greatly the polarization characteristics. A variation in characteristics can be improved by relaxing the stress variation.
According to the comparative experiments made by the inventor, in a structure (so-called junction down mount) that a second electrode formed on a multilayer semiconductor layer including an active layer is stacked upon and bonded to a submount by using a soft bonding material such as solder, stresses particularly stresses near the mount plane of the semiconductor laser chip are dominant. A semiconductor laser diode of a ridge structure has a structure having a separation groove on both sides of a ridge, and two cavities corresponding to the separation grooves appear on the surface of the second electrode. Therefore, when the semiconductor laser is mounted on a submount (supporting base plate) by using a bonding material, it is difficult for the bonding material to reliably enter the cavities corresponding to the separation grooves, so that voids are generated at the interface between the second electrode plane and bonding material. It has been clarified that these voids apply stresses to an active layer region corresponding to the ridge, in other words, a resonator region, and a variation in polarization angles occurs. Namely, as voids are generated, uniformity (balance) of stresses applied to the ridge region from the submount and bonding material is broken, and rotation of a polarization angle occurs.
FIGS. 19A and 19B are schematic diagrams showing a state of mounting on a submount substrate a semiconductor laser diode of a ridge structure studied prior to the present invention. FIG. 19A is a diagram showing a state before the semiconductor laser diode is mounted on the submount, and FIG. 19B is a diagram showing a state after the semiconductor laser diode is mounted on the submount.
FIG. 19A shows a submount 70 and a semiconductor laser diode 75 to be mounted on a first plane 70a of the submount 70. The submount 70 is a flat plate body made of aluminum nitride (AlN) or the like, and an electrode pad 71 having a predetermined pattern is disposed on the first plane 70a. The electrode pad 71 is made of, e.g., an Au layer having a thickness of about 0.5 to 1.0 μm. A soft bonding material 72 is formed on the electrode pad 71. The bonding material 71 is made of, e.g., solder such as AuSn having a thickness of about 3 to 5 μm.
In order to mount the semiconductor laser diode 75 on the submount 70 in junction down, there is shown in FIGS. 19A and 19B a state that a first plane 76a of a semiconductor substrate 76 of a first conductivity constituting the semiconductor laser diode 75 is positioned at a lower plane, and that a second plane 76b as an opposite plane of the first plane 76a is positioned at an upper plane. On the first plane 76a of the semiconductor substrate 76, a multilayer semiconductor layer is formed by sequentially laminating a first conductivity type clad layer 77 of the first conductivity type, an active layer 78, a second conductivity type clad layer 79 of a second conductivity type and a contact layer 80 of the second conductivity type.
On the surface of the multilayer semiconductor layer, three or more partitioning groves 81 are formed extending from one end to the other of the semiconductor substrate 76 (e.g., from a front side to the back side of the drawing sheet) and from the surface of the contact layer 80 to a predetermined depth of the second conductivity type clad layer 79. In the drawing, two partitioning groove 81 are shown.
Two separation grooves 83 are formed in a central region of a partitioning region 82 sandwiched between the adjacent partitioning grooves 81. Similar to the partitioning groove 81, the separation groove 83 is formed extending from one end to the other of the semiconductor substrate 76 and from the surface of the contact layer 80 to the predetermined depth of the second conductivity type clad layer 79. A stripe-shaped convex region sandwiched between two separation grooves 83 is a ridge 84. Multilayer semiconductor layer regions outside the separation grooves 83, i.e., regions sandwiched between the separation grooves 83 and partitioning grooves 81 are called a field region 85.
On both sides of the ridge 84, an insulating layer 86 is formed covering the area from each side wall of the contact layer 80 of the ridge 84, via the separation groove 83 to the end of the partitioning region 82. A first electrode 91 is formed on the second plane 76b of the semiconductor substrate 76. The first electrode 91 is constituted of a first electrode layer 92 as an underlying layer and a first plating layer 93 formed on the first electrode layer 92.
On the side of the first plane 76a of the semiconductor substrate 76, a second electrode 95 is formed. The second electrode 95 is formed above the surface of the partitioning region 82, covering the areas above the ridge 84, above the separation groove 83 on both sides of the ridge 84, and above the field regions 85 (multilayer semiconductor layer regions outside the separation grooves 83). The second electrode 95 is constituted of a second electrode layer 96 as an underlying layer and a second plating layer 97 formed on the second electrode layer 96. The first and second electrode layers 92 and 96 are each a layer formed by sequentially laminating a Ti layer, a Pt layer and an Au layer by evaporation, and are each made of Au having a thickness of about 0.3 to 0.5 μm. The first and second plated layers 93 and 97 are each an Au layer having a thickness of about 3 to 5 μm formed by a plating process.
The semiconductor laser diode 75 shown in FIG. 19A is held by an unrepresented holding tool such as a collet, positioned above the submount 70, and lowered as indicated by an arrow to be stacked upon the submount 70. The semiconductor laser diode 75 is therefore mounted on (fixed to) the submount 70, as shown in FIG. 19B. During fixing, since the grooves are formed on the surface of the second electrode 95 in correspondence with the separation grooves 83, there is a high possibility that air remains in the grooves when the second electrode 95 is pushed against the bonding material 72, even if the melted bonding material 72 invades into the grooves. Remaining air forms an air bubble (void) 98. Since the semiconductor laser diode 75 generates heat during laser oscillation, a high temperature state of about 80° C. occurs. As the air bubble 98 is generated at the interface between the second electrode 95 and bonding material 72, uniformity (balance) of stresses applied to the ridge 84 having a narrow width of about 1 to 3 μmn is broken, resulting in rotation of a polarization angle. As shown in FIG. 19B, stresses indicated by arrows are applied, for example, to the center of the ridge 84 from both sides of the ridge 84, depending upon a difference of thermal expansion coefficients of various materials. In this case, in the region where the air bubble (void) 98 is generated, the air bubble region functions as a buffer, so that a stress indicated by an arrow directing from right to left is smaller than a stress indicated by an arrow directing from left to right. The arrow directing from right to left is shown shorter than the arrow directing from left to right, indicating a smaller stress. As uniformity (balance) of stress is broken, rotation of a polarization angle occurs.
FIG. 18 is a schematic diagram showing a polarization angle α. FIG. 18 is a plan view showing a state that a cap fixed to a first plane of a stem 100 is dismounted. A heat sink 101 having a rectangular solid shape is fixed to the central area of the first plane of the stem 100. A submount 70 is fixed to the side wall of the heat sink 101 on the center side of the stem 100. A laser beam 102 is indicated by a black circle and emitted from a facet (emission plane) of the semiconductor laser diode 75. A plane having arrows at opposite ends in FIG. 18 is a polarization plane 103, and an angle between the polarization plane 103 and a plane along an extension direction of an unrepresented active layer of the semiconductor laser diode 75 is the polarization angle α.
Even a semiconductor laser diode having a single resonator, if semiconductor laser diodes have a variation in polarization angles, the semiconductor laser diodes cannot be used, because desired characteristics cannot be obtained if an optical component has polarization angle dependency.
In the case of a semiconductor laser diode of a multibeam structure that the semiconductor laser diode has a plurality of resonators disposed in parallel, if there is a variation in polarization angles of laser beams emitted from respective resonators, the semiconductor laser diode cannot be used for an electronic apparatus such as a plain paper copier (PPC) and a laser beam printer using a semiconductor laser diode of the multibeam structure as its light source.
The semiconductor laser of JP-A-2005-217255 has a structure that the upper surface of the plated metal layer formed on the upper surface side of the ridge stripe region is planarized in order to prevent heat radiation from being lowered by generation of a void. However, if the structure of the flat upper surface of the plated metal layer has a structure that the flat plane of the plated metal layer is stacked upon the solder on the heat sink and the semiconductor laser is fixed by heating, melting and bonding, there is specifically no function of pushing air existing at the interface in the stacked state to the peripheral edges of the plated metal layer. Therefore, air may be left and an air bubble (void) is generated at the bonding interface. Further, although the surface of the plated metal layer is flat, if the surface of the solder is flat, the flat plane is stacked upon the flat plane so that an area not wetting the solder may be formed depending upon manufacture variation.
In a semiconductor laser diode of the multibeam structure that a plurality of resonators exist in parallel on the semiconductor laser diode, there are as many bonding regions by solder between the submount and electrode as the number of resonators so that a possibility of generating an air bubble (void) becomes high. A polarization angle of a laser beam emitted from the resonator having a generated air bubble becomes different from that of the other laser beams. This semiconductor laser diode cannot be used as a light source of an electronic apparatus having an optical component with polarization angle dependency.